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Magnetocaloric effect in the La0.62Bi0.05Ca0.33MnO3 compound
ARTICLE IN PRESS
Physica B 357 (2005) 326–333 www.elsevier.com/locate/physb
Magnetocaloric effect in the La0.62Bi0.05Ca0.33MnO3 compound H. Gencer, S. Atalay, H.I. Adiguzel, V.S. Kolat Department of Physics, Science and Arts Faculty, Inonu University, 44069 Malatya, Turkey Received 25 October 2004; received in revised form 25 November 2004; accepted 26 November 2004
Abstract Bi-doped lanthanum manganite with chemical composition of La0.62Bi0.05Ca0.33MnO3 has been prepared by the standard solid-state process. The effect of Bi doping on the sintering temperature, the magnetic and magnetocaloric properties has been investigated. Bi2O3 was found to be an effective sintering aid for La0.67Ca0.33MnO3 . It was found that small amounts of Bi doping (x ¼ 0:05) can lower the sintering temperature about 200 1C. Bi-doped samples also exhibited a large magnetic entropy change, |DSm| ¼ 3.5 J/kg K under 1 T field at the Curie temperature. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.30.Sg Keywords: Magnetocaloric effect; Manganites
1. Introduction The temperature change of a magnetic material associated with an external magnetic field change is defined as a magnetocaloric (MC) effect which was first discovered in 1881 by Werburg [1]. In recent years, due to the possibility of using magnetic materials as an active magnetic refrigerant in magnetic refrigeration technology, research on the magnetocaloric properties of magnetic Corresponding author. Tel.: +90 422 3410010;
fax: +90 422 3410037. E-mail address: [email protected] (S. Atalay).
materials has attracted considerable attention [2–5]. Two of the main goals of recent studies on MC effect is to seek out and find the most useful magnetic materials which have a large entropy change at low magnetic fields and their possibility of application from low temperature to near the room temperature. In the last few years, the large magnetic entropy change has been intensively studied in perovskite manganites and has attracted attention for their considerable advantages in magnetic refrigeration technology, such as high efficiency, non-pollution, good chemical stability, tuneable ordering temperature, low working field and low cost [6–10].
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.11.084
ARTICLE IN PRESS H. Gencer et al. / Physica B 357 (2005) 326–333
There are two important requirements for a magnetic material to possess a large magnetic entropy change. One is a large enough spontaneous magnetisation and the other is a sharp drop in the magnetisation with increasing temperature associated with the ferromagnetic (FM) to paramagnetic (PM) transition at the Curie temperature (Tc). The large value of the spontaneous magnetisation in LaCaMnO and its abrupt drop at Tc, make this compound an attractive candidate for a large magnetocaloric effect. There have been many studies to improve the MC effect in LaCaMnO [11–16]. One way is the replacement of the La3+ ion in perovskite manganites with a doping element which has different ionic size and oxidation state [17–20]. The most interesting results have been observed with Bi3+ substitution for La3+ both of which have a very similar ionic radius and the same oxidation state. In this case, neither average ionic radii (/rAS) of the A-site nor the content of Mn3+ (charge balance) is expected to vary, and both of these properties are dominant effects on magnetic and electrical properties. However, previous studies on the Bi-doped perovskite manganites have revealed that Bi-doped compounds have quite different electrical, magnetic and magnetoresistance properties [21–24]. Interestingly there has been no report about Bi-doping effects on magnetocaloric properties for LaCaMnO. Therefore in this work, we have studied the effect of Bi doping on magnetic and magnetocaloric properties of La0.67Ca0.33MnO3.
2. Experimental details The polycrystalline La0.62Bi0.05Ca0.33MnO3 sample was prepared by conventional solid-state reaction using high purity powders La2O3, CaCO3, MnO. The powders were mixed, ground thoroughly and pre-sintered in air at 800 1C for 10 h. Then the sample was ground and shaped by press and sintered at 1200 1C for 24 h in air. The structural characterisation was carried out using X-ray powder diffraction (Cu Ka). Grain structure was observed using a LEO-EVO-40 scanning electron microscope (SEM). The magnetic measurements were performed using a Q-3398 (cryo-
327
genic) magnetometer in a temperature range from 150 to 300 K and a maximum magnetic field of 6 T was applied.
3. Results and discussion 3.1. Bi as a sintering aid The results of the X-ray diffraction indicate that the substitution of La by Bi does not produce any significant structural change (Fig. 1). The X-ray patterns of La0.62Bi0.05Ca0.33MnO3 sample are consistent with a cubic perovskite-type structure with a lattice constant of 3.8586 A˚. Bi substitution leads to a very small decrease in the lattice ( for constant from a reported value of a ¼ 3:863 A La0.67Ca0.33MnO3 [23], since the ionic radius for Bi is nearly equal to that of La. Fig. 2 shows the SEM photographs for the La0.62Bi0.05Ca0.33MnO3 sample sintered at 1200 1C. The SEM images reflect a polycrystalline structure with grain sizes between 4 and 10 mm. Recently, Kim et al. [24] and Schiffer et al. [25] have shown that the size of the crystal grains of a La0.67Ca0.33MnO3 sample sintered at 1200 1C are smaller than those of sample sintered at the usual sintering temperature of 1400 1C. This indicates that 1200 1C is not enough for good crystallinity for La0.67Ca0.33MnO3. However, our experimental results show that the size of the crystal grains for La0.62Bi0.05Ca0.33MnO3 sintered at 1200 1C agrees well with those of a La0.67Ca0.33MnO3 sample sintered at 1400 1C [25]. This result indicates that Bi doping promotes sintering due to its low melting temperature resulting in a good crystallinity of sample even at 1200 1C. With the addition of Bi2O3 in LaCaMnO3, the Bi3+ ions enter into the A (La3+)-sites and increase the A/B ratio and create B(Mn)-site vacancies in the material. The La3+ and Ca2+ (A-site) ions with larger ionic radii are reported to be slower moving species than Mn3+ and Mn4+ (B-site) in the compound [26]. The diffusivity of the Bi3+ ion with smaller ionic radius is faster than that of La3+ and Ca2+ ions. Thus, increase in Bi3+ ions as a result of addition of Bi2O3 increases the diffusivity of A-site ions and thereby enhances the sintering process. If Bi is substituted for La, the
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(011)
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400
300
200 (112)
(002) (111)
100
(022)
(001)
(013)
0 20
30
40
50
60
70
80
Fig. 1. The X-ray diffraction patterns of La0.62Bi0.05Ca0.33MnO3.
material can sinter at relatively low temperatures as observed in our experimental results. Under this procedure, the preparation of samples is easier than undoped samples. 3.2. Effect of Bi on magnetic and magnetocaloric properties The temperature dependence of magnetisation for the La0.62Bi0.05Ca0.33MnO3 sample is shown in Fig. 3. The Curie temperature Tc, defined as the temperature of the maximum value in |dM/dT|, was measured to be 248 K for La0.62Bi0.05Ca0.33MnO3 sample. The Curie temperature of the Bidoped sample was found to be slightly lower than that of the undoped sample [25]. This indicates that Bi substitution appears to weaken the magnetic interaction in the sample. Fig. 4. shows the plots of magnetisation against the applied field (from 0 to 6 T) obtained at various temperatures. Fig. 5 shows Arrott plots of the La0.62Bi0.05 Ca0.33MnO3 sample. A negative slope in the isotherm plots of H/M versus M2 is a clear indication of a first-order phase transition from ferromagnetism to paramagnetism. The magnetic entropy, which is associated with the MC effect, can be calculated from the
isothermal magnetisation curves (Fig. 4) under the influence of a magnetic field. According to the classical thermodynamical theory, the magnetic entropy change DS m produced by the variation of a magnetic field from 0 to Hmax is given by Z H max qS DS m ðT; HÞ ¼ dH: (1) qH T 0 From Maxwell’s thermodynamic relation: qS qM ¼ ; qH T qT H one can obtain the following expression: Z H max qM dH: DS m ðT; HÞ ¼ qT H 0
(2)
(3)
In order to evaluate the magnetic entropy change DSm one needs to make a numerical approximation to the integral in Eq. (3). The usual method is to use isothermal magnetisation measurements at small discrete field intervals, jDS m j can be approximated from Eq. (3) by X M i M iþ1 jDS m j ¼ DH; (4) T iþ1 T i i
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Fig. 2. The SEM photographs of La0.62Bi0.05Ca0.33MnO3 with a magnification of (a) 1000 and (b) 5000.
where Mi and Mi+1 are the experimental values of the magnetisation at Ti and Ti+1, respectively, under an applied magnetic field Hi. Using Eq. (4), by measuring the M–H curve at various temperatures, one can calculate the magnetic entropy change associated with the magnetic field variation. The entropy change for the La0.62Bi0.05-
Ca0.33MnO3 sample calculated using Eq. (4) as a function of temperature is given in Fig. 6. A peak of jDS m j around the Curie temperature was observed. The maximum entropy change corresponding to a magnetic field variation of 1 T is about 3.5 J kg K which is slightly higher than that of La0.67Ca0.33MnO3 reported in the literature
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T (K)
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T (K) Fig. 3. Temperature dependence of magnetisation for La0.62Bi0.05Ca0.33MnO3 at different magnetic fields. Inset shows Curie temperature of sample, where measurement was performed at zero magnetic field.
220 K 229 K 238 K 243 K 248 K 253 K 258 K 262 K 267 K 272 K 277 K 282 K 286 K 291 K 295 K
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6
H (T)
Fig. 4. Magnetic field dependence of the magnetisation measured at different temperatures for La0.62Bi0.05Ca0.33MnO3.
[6,12]. This indicates that a small amount of Bi3+ doping favours the enhancement of the magnetic entropy change, and consequently the MC effect in this material. In perovskite manganites, there are two main mechanisms which give the magnetic properties to
a material. One is antiferromagnetic super-exchange interaction in the Mn3+–O2–Mn3+ (or Mn4+–O2–Mn4+) bonds and the other is FM double-exchange interaction in Mn3+–O2Mn4+ bonds. There are many factors that cause in the strength of these interactions. In many studies on
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0.25
295 K 286 K 282 K 277 K 272 K 267 K 262 K 258 K 253 K 248 K 243 K 238 K 229 K 220 K
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Fig. 5. The M2 versus H/M (Arrott) plots of La0.62Bi0.05Ca0.33MnO3.
Fig. 6. Temperature dependence of the magnetic entropy change jDSm j for La0.62Bi0.05Ca0.33MnO3.
the perovskite manganites, it has been shown that the replacement of A-site ions with others which have different size and oxidation states can cause two main changes in the materials. The Mn3+/ Mn4+ ratio and structural change cause a change in Mn–O bond length and Mn–O–Mn bond angle [6,10,17]. Both of them cause the change of the
above-mentioned magnetic interaction and, consequently, the magnetic properties of the sample. In La0.67Ca0.33MnO3, by replacing the La3+ ion with Bi3+ ion, neither the structural change nor the charge balance (Mn3+/Mn4+) is expected to vary because of both La and Bi ions have the same 3+ oxidation state and very similar ionic radius
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( and r ¼ 1:300 A). ( Our experimen(rLa ¼ 1:302 A Bi tal results show that the doping of La0.67Ca0.33MnO3 with a low amount of Bi ions gives rise to changes in magnetic (as seen from decrease of Curie temperature) and MC effect (as seen from increase of magnetic entropy) properties of these materials. The interesting point here is what causes such a change of magnetic and MC properties despite the equivalence of Bi3+ and La3+. It has been shown that the Bi–O bond is shorter than the La–O bond despite the similar ionic radii of Bi3+ and La3+ ions [22]. This can be interpreted as arising from the rather covalent character of the Bi–O bonds. The electronegativity of Bi enhances an hybridisation between 6 s of Bi3+ orbitals and 2p of O2 orbitals and can produce a local distortion. The measured 1591 of Mn–O–Mn bond angle for a Bi-doped sample [27], which is smaller than that of an undoped sample, is evidence of this hybridisation and local distortion. This local distortion can hinder the movement of the eg electron from Mn to Mn. This causes localisation of electrons around Bi ions and weakens the FM double-exchange interactions and increases the antiferromagnetic super exchange interactions. This fact is reflected in the decrease of Curie temperature. For the above reasons, FM coupling weakens and Tc drops. As shown in Fig. 6, Bi3+ doping favours the enhancement of magnetic entropy change. Fig. 3 shows the saturation magnetisation and its variation versus temperature for La0.62Bi0.05Ca0.33MnO3. The value of saturation magnetisation of the Bi-doped sample is nearly equal to that of La0.67Ca0.33MnO3 [25]. The magnetisation variation near the Tc is faster than in the undoped sample. Therefore, the Bi-doped sample has a slightly higher entropy change than undoped sample. La0.67xBixCa0.33MnO3 sample with various x is in preparation in order to investigate more fully the effect of Bi doping on the MC effect on this compound.
4. Conclusions Bi-doped lanthanum manganite with chemical composition of La0.62Bi0.05Ca0.33MnO3 has been
prepared by the standard solid-state method. The effect of Bi doping on sintering temperature, magnetic and magnetocaloric properties has been investigated. Bi2O3 was found to be an effective sintering aid for La0.67Ca0.33MnO3. As shown in the SEM photograph, a good crystallinity has been observed for the Bi-doped sample even when it was sintered at 1200 1C, which is about 200 1C lower than the normal sintering temperature of an undoped sample. The Bi-doped sample also exhibited a lower Curie temperature than for an undoped sample. The decrease of Curie temperature was discussed in terms of local distortion from the covalent character of the Bi–O bond. The other important result that was observed was a large magnetic entropy change which is about 3.5 J/kg K under the effect of 1 T magnetic field near the Curie temperature for La0.62Bi0.05Ca0.33MnO3 sample.
Acknowledgement This work was supported by DPT and TUBA, in the frame work of the Young Scientist Award Program (SA/TUBA-GEBIP/2002-1-3). References [1] E. Werburg, Ann. Phys. Chem. 13 (1881) 141. [2] G.V. Brown, J. Appl. Phys. 47 (1976) 3673. [3] M. Foldeaki, R. Chahine, T.K. Bore, J. Appl. Phys. 77 (1995) 3528. [4] V.K. Pecharsky, K.A. Gschneider, Phys. Rev. Lett. 78 (1997) 4494. [5] O. Tegus, E. Bru¨ck, K.H.J. Buschow, F.R. de Boer, Nature 415 (2002) 150. [6] X.X. Zhang, J. Tejeda, Y. Xin, G.F. Sun, K.W. Wong, X. Bohigas, Appl. Phys. Lett. 69 (1996) 3596. [7] K. Huang, Z.B. Guo, D.H. Wang, T.W. Du, J. Magn. Magn. Mater. 173 (1997) 302. [8] X. Bohigas, J. Tejeda, E. Barco, X.X. Zhang, M. Sales, Appl. Phys. Lett. 73 (1998) 390. [9] A. Szewczyk, H. Szymczak, A. Wisnieski, K. Piotrowski, Appl. Phys. Lett. 77 (2000) 1026. [10] Y. Sun, W. Tong, N. Liu, Y.H. Zhang, J. Magn. Magn. Mater. 238 (2002) 25. [11] P. Sanda, L.E. Hueso, D.R. Miguens, J. Rives, Appl. Phys. Lett. 79 (2001) 2040. [12] H. Chen, C. Lin, D. Dai, J. Magn. Magn. Mater. 257 (2003) 254.
ARTICLE IN PRESS H. Gencer et al. / Physica B 357 (2005) 326–333 [13] A.M. Gomes, M.S. Reis, A.P. Guimaraes, J. Magn. Magn. Mater. 272 (2004) 2385. [14] A.N. Ulyenov, I.S. Maksimov, E.B. Nyeanchi, S.C. Yu, J. Magn. Magn. Mater. 258 (2003) 312. [15] L.E. Hueso, J. Rives, J. Appl. Phys. 86 (1999) 3881. [16] P.A. Joy, P.S. Kumar, S.K. Date, Mater. Res. Bull. 34 (1999) 2143. [17] P.G. Redealli, M. Marezio, H.Y. Huang, S.W. Cheong, B. Batlegg, Phys. Rev. B 54 (1996) 8992. [18] S.B. Tian, M.H. Phan, S.C. Yu, N.H. Hur, Physica B 327 (2003) 221. [19] Z.B. Guo, Y.W. Du, J.S. Zhu, H. Huang, W.P. Ding, D. Feng, Phys. Rev. Lett. 781 (1997) 1142. [20] J.B. Goodenough, Phys. Rev. 100 (1955) 564.
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[21] L. Righi, J. Gutierrez, J.M. Barandiaran, J. Phys.: Condens. Matter 11 (1999) 2831. [22] L. RI˙ghai, M. Amboage, J. Gutierrez, J.M. Barandiaran, L.F. Barquin, M.T.F. Diaz, Physica B 276–278 (2000) 718. [23] G. Srinivasen, R.M. Savage, V.S. Babu, M.S. Seehra, J. Magn. Magn. Mater. 168 (1997) 1. [24] D. Kim, J. Cho, J. Matt. Science 35 (2000) 5939. [25] P. Schiffer, A.P. Ramirez, W. Bao, S.W. Cheong, Phys. Rev. Lett. 75 (1995) 3336. [26] J.A.M. Van Roosmalen, E.H.P. Cordfunke, J.P.P. Huijsmans, Solid State Ionics 66 (1993) 285. [27] J.M. Barandiaran, J. Gutierrez, J.R. Fernandez, M. Amboage, L. Righi, Physica B 343 (2004) 379.
Physica B 357 (2005) 326–333 www.elsevier.com/locate/physb
Magnetocaloric effect in the La0.62Bi0.05Ca0.33MnO3 compound H. Gencer, S. Atalay, H.I. Adiguzel, V.S. Kolat Department of Physics, Science and Arts Faculty, Inonu University, 44069 Malatya, Turkey Received 25 October 2004; received in revised form 25 November 2004; accepted 26 November 2004
Abstract Bi-doped lanthanum manganite with chemical composition of La0.62Bi0.05Ca0.33MnO3 has been prepared by the standard solid-state process. The effect of Bi doping on the sintering temperature, the magnetic and magnetocaloric properties has been investigated. Bi2O3 was found to be an effective sintering aid for La0.67Ca0.33MnO3 . It was found that small amounts of Bi doping (x ¼ 0:05) can lower the sintering temperature about 200 1C. Bi-doped samples also exhibited a large magnetic entropy change, |DSm| ¼ 3.5 J/kg K under 1 T field at the Curie temperature. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.30.Sg Keywords: Magnetocaloric effect; Manganites
1. Introduction The temperature change of a magnetic material associated with an external magnetic field change is defined as a magnetocaloric (MC) effect which was first discovered in 1881 by Werburg [1]. In recent years, due to the possibility of using magnetic materials as an active magnetic refrigerant in magnetic refrigeration technology, research on the magnetocaloric properties of magnetic Corresponding author. Tel.: +90 422 3410010;
fax: +90 422 3410037. E-mail address: [email protected] (S. Atalay).
materials has attracted considerable attention [2–5]. Two of the main goals of recent studies on MC effect is to seek out and find the most useful magnetic materials which have a large entropy change at low magnetic fields and their possibility of application from low temperature to near the room temperature. In the last few years, the large magnetic entropy change has been intensively studied in perovskite manganites and has attracted attention for their considerable advantages in magnetic refrigeration technology, such as high efficiency, non-pollution, good chemical stability, tuneable ordering temperature, low working field and low cost [6–10].
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.11.084
ARTICLE IN PRESS H. Gencer et al. / Physica B 357 (2005) 326–333
There are two important requirements for a magnetic material to possess a large magnetic entropy change. One is a large enough spontaneous magnetisation and the other is a sharp drop in the magnetisation with increasing temperature associated with the ferromagnetic (FM) to paramagnetic (PM) transition at the Curie temperature (Tc). The large value of the spontaneous magnetisation in LaCaMnO and its abrupt drop at Tc, make this compound an attractive candidate for a large magnetocaloric effect. There have been many studies to improve the MC effect in LaCaMnO [11–16]. One way is the replacement of the La3+ ion in perovskite manganites with a doping element which has different ionic size and oxidation state [17–20]. The most interesting results have been observed with Bi3+ substitution for La3+ both of which have a very similar ionic radius and the same oxidation state. In this case, neither average ionic radii (/rAS) of the A-site nor the content of Mn3+ (charge balance) is expected to vary, and both of these properties are dominant effects on magnetic and electrical properties. However, previous studies on the Bi-doped perovskite manganites have revealed that Bi-doped compounds have quite different electrical, magnetic and magnetoresistance properties [21–24]. Interestingly there has been no report about Bi-doping effects on magnetocaloric properties for LaCaMnO. Therefore in this work, we have studied the effect of Bi doping on magnetic and magnetocaloric properties of La0.67Ca0.33MnO3.
2. Experimental details The polycrystalline La0.62Bi0.05Ca0.33MnO3 sample was prepared by conventional solid-state reaction using high purity powders La2O3, CaCO3, MnO. The powders were mixed, ground thoroughly and pre-sintered in air at 800 1C for 10 h. Then the sample was ground and shaped by press and sintered at 1200 1C for 24 h in air. The structural characterisation was carried out using X-ray powder diffraction (Cu Ka). Grain structure was observed using a LEO-EVO-40 scanning electron microscope (SEM). The magnetic measurements were performed using a Q-3398 (cryo-
327
genic) magnetometer in a temperature range from 150 to 300 K and a maximum magnetic field of 6 T was applied.
3. Results and discussion 3.1. Bi as a sintering aid The results of the X-ray diffraction indicate that the substitution of La by Bi does not produce any significant structural change (Fig. 1). The X-ray patterns of La0.62Bi0.05Ca0.33MnO3 sample are consistent with a cubic perovskite-type structure with a lattice constant of 3.8586 A˚. Bi substitution leads to a very small decrease in the lattice ( for constant from a reported value of a ¼ 3:863 A La0.67Ca0.33MnO3 [23], since the ionic radius for Bi is nearly equal to that of La. Fig. 2 shows the SEM photographs for the La0.62Bi0.05Ca0.33MnO3 sample sintered at 1200 1C. The SEM images reflect a polycrystalline structure with grain sizes between 4 and 10 mm. Recently, Kim et al. [24] and Schiffer et al. [25] have shown that the size of the crystal grains of a La0.67Ca0.33MnO3 sample sintered at 1200 1C are smaller than those of sample sintered at the usual sintering temperature of 1400 1C. This indicates that 1200 1C is not enough for good crystallinity for La0.67Ca0.33MnO3. However, our experimental results show that the size of the crystal grains for La0.62Bi0.05Ca0.33MnO3 sintered at 1200 1C agrees well with those of a La0.67Ca0.33MnO3 sample sintered at 1400 1C [25]. This result indicates that Bi doping promotes sintering due to its low melting temperature resulting in a good crystallinity of sample even at 1200 1C. With the addition of Bi2O3 in LaCaMnO3, the Bi3+ ions enter into the A (La3+)-sites and increase the A/B ratio and create B(Mn)-site vacancies in the material. The La3+ and Ca2+ (A-site) ions with larger ionic radii are reported to be slower moving species than Mn3+ and Mn4+ (B-site) in the compound [26]. The diffusivity of the Bi3+ ion with smaller ionic radius is faster than that of La3+ and Ca2+ ions. Thus, increase in Bi3+ ions as a result of addition of Bi2O3 increases the diffusivity of A-site ions and thereby enhances the sintering process. If Bi is substituted for La, the
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(011)
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500
400
300
200 (112)
(002) (111)
100
(022)
(001)
(013)
0 20
30
40
50
60
70
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Fig. 1. The X-ray diffraction patterns of La0.62Bi0.05Ca0.33MnO3.
material can sinter at relatively low temperatures as observed in our experimental results. Under this procedure, the preparation of samples is easier than undoped samples. 3.2. Effect of Bi on magnetic and magnetocaloric properties The temperature dependence of magnetisation for the La0.62Bi0.05Ca0.33MnO3 sample is shown in Fig. 3. The Curie temperature Tc, defined as the temperature of the maximum value in |dM/dT|, was measured to be 248 K for La0.62Bi0.05Ca0.33MnO3 sample. The Curie temperature of the Bidoped sample was found to be slightly lower than that of the undoped sample [25]. This indicates that Bi substitution appears to weaken the magnetic interaction in the sample. Fig. 4. shows the plots of magnetisation against the applied field (from 0 to 6 T) obtained at various temperatures. Fig. 5 shows Arrott plots of the La0.62Bi0.05 Ca0.33MnO3 sample. A negative slope in the isotherm plots of H/M versus M2 is a clear indication of a first-order phase transition from ferromagnetism to paramagnetism. The magnetic entropy, which is associated with the MC effect, can be calculated from the
isothermal magnetisation curves (Fig. 4) under the influence of a magnetic field. According to the classical thermodynamical theory, the magnetic entropy change DS m produced by the variation of a magnetic field from 0 to Hmax is given by Z H max qS DS m ðT; HÞ ¼ dH: (1) qH T 0 From Maxwell’s thermodynamic relation: qS qM ¼ ; qH T qT H one can obtain the following expression: Z H max qM dH: DS m ðT; HÞ ¼ qT H 0
(2)
(3)
In order to evaluate the magnetic entropy change DSm one needs to make a numerical approximation to the integral in Eq. (3). The usual method is to use isothermal magnetisation measurements at small discrete field intervals, jDS m j can be approximated from Eq. (3) by X M i M iþ1 jDS m j ¼ DH; (4) T iþ1 T i i
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Fig. 2. The SEM photographs of La0.62Bi0.05Ca0.33MnO3 with a magnification of (a) 1000 and (b) 5000.
where Mi and Mi+1 are the experimental values of the magnetisation at Ti and Ti+1, respectively, under an applied magnetic field Hi. Using Eq. (4), by measuring the M–H curve at various temperatures, one can calculate the magnetic entropy change associated with the magnetic field variation. The entropy change for the La0.62Bi0.05-
Ca0.33MnO3 sample calculated using Eq. (4) as a function of temperature is given in Fig. 6. A peak of jDS m j around the Curie temperature was observed. The maximum entropy change corresponding to a magnetic field variation of 1 T is about 3.5 J kg K which is slightly higher than that of La0.67Ca0.33MnO3 reported in the literature
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T (K) Fig. 3. Temperature dependence of magnetisation for La0.62Bi0.05Ca0.33MnO3 at different magnetic fields. Inset shows Curie temperature of sample, where measurement was performed at zero magnetic field.
220 K 229 K 238 K 243 K 248 K 253 K 258 K 262 K 267 K 272 K 277 K 282 K 286 K 291 K 295 K
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2
3
4
5
6
H (T)
Fig. 4. Magnetic field dependence of the magnetisation measured at different temperatures for La0.62Bi0.05Ca0.33MnO3.
[6,12]. This indicates that a small amount of Bi3+ doping favours the enhancement of the magnetic entropy change, and consequently the MC effect in this material. In perovskite manganites, there are two main mechanisms which give the magnetic properties to
a material. One is antiferromagnetic super-exchange interaction in the Mn3+–O2–Mn3+ (or Mn4+–O2–Mn4+) bonds and the other is FM double-exchange interaction in Mn3+–O2Mn4+ bonds. There are many factors that cause in the strength of these interactions. In many studies on
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0.25
295 K 286 K 282 K 277 K 272 K 267 K 262 K 258 K 253 K 248 K 243 K 238 K 229 K 220 K
H / M (T g/emu)
0.20
0.15
0.10
0.05
0.00 0
1000
2000
3000
M2
4000
5000
(emu/g)2
Fig. 5. The M2 versus H/M (Arrott) plots of La0.62Bi0.05Ca0.33MnO3.
Fig. 6. Temperature dependence of the magnetic entropy change jDSm j for La0.62Bi0.05Ca0.33MnO3.
the perovskite manganites, it has been shown that the replacement of A-site ions with others which have different size and oxidation states can cause two main changes in the materials. The Mn3+/ Mn4+ ratio and structural change cause a change in Mn–O bond length and Mn–O–Mn bond angle [6,10,17]. Both of them cause the change of the
above-mentioned magnetic interaction and, consequently, the magnetic properties of the sample. In La0.67Ca0.33MnO3, by replacing the La3+ ion with Bi3+ ion, neither the structural change nor the charge balance (Mn3+/Mn4+) is expected to vary because of both La and Bi ions have the same 3+ oxidation state and very similar ionic radius
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( and r ¼ 1:300 A). ( Our experimen(rLa ¼ 1:302 A Bi tal results show that the doping of La0.67Ca0.33MnO3 with a low amount of Bi ions gives rise to changes in magnetic (as seen from decrease of Curie temperature) and MC effect (as seen from increase of magnetic entropy) properties of these materials. The interesting point here is what causes such a change of magnetic and MC properties despite the equivalence of Bi3+ and La3+. It has been shown that the Bi–O bond is shorter than the La–O bond despite the similar ionic radii of Bi3+ and La3+ ions [22]. This can be interpreted as arising from the rather covalent character of the Bi–O bonds. The electronegativity of Bi enhances an hybridisation between 6 s of Bi3+ orbitals and 2p of O2 orbitals and can produce a local distortion. The measured 1591 of Mn–O–Mn bond angle for a Bi-doped sample [27], which is smaller than that of an undoped sample, is evidence of this hybridisation and local distortion. This local distortion can hinder the movement of the eg electron from Mn to Mn. This causes localisation of electrons around Bi ions and weakens the FM double-exchange interactions and increases the antiferromagnetic super exchange interactions. This fact is reflected in the decrease of Curie temperature. For the above reasons, FM coupling weakens and Tc drops. As shown in Fig. 6, Bi3+ doping favours the enhancement of magnetic entropy change. Fig. 3 shows the saturation magnetisation and its variation versus temperature for La0.62Bi0.05Ca0.33MnO3. The value of saturation magnetisation of the Bi-doped sample is nearly equal to that of La0.67Ca0.33MnO3 [25]. The magnetisation variation near the Tc is faster than in the undoped sample. Therefore, the Bi-doped sample has a slightly higher entropy change than undoped sample. La0.67xBixCa0.33MnO3 sample with various x is in preparation in order to investigate more fully the effect of Bi doping on the MC effect on this compound.
4. Conclusions Bi-doped lanthanum manganite with chemical composition of La0.62Bi0.05Ca0.33MnO3 has been
prepared by the standard solid-state method. The effect of Bi doping on sintering temperature, magnetic and magnetocaloric properties has been investigated. Bi2O3 was found to be an effective sintering aid for La0.67Ca0.33MnO3. As shown in the SEM photograph, a good crystallinity has been observed for the Bi-doped sample even when it was sintered at 1200 1C, which is about 200 1C lower than the normal sintering temperature of an undoped sample. The Bi-doped sample also exhibited a lower Curie temperature than for an undoped sample. The decrease of Curie temperature was discussed in terms of local distortion from the covalent character of the Bi–O bond. The other important result that was observed was a large magnetic entropy change which is about 3.5 J/kg K under the effect of 1 T magnetic field near the Curie temperature for La0.62Bi0.05Ca0.33MnO3 sample.
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